1. Technical Field
The present invention relates to a light source device and an image display device.
2. Related Art
Discharge lamps such as extra-high pressure mercury lamps are generally used as light sources in projection type image display device in recent years.
However, in such a discharge lamp, there are problems in that the service lifetime is short, instantaneous lighting is difficult, the color reproducibility range is narrow, and the ultraviolet rays emitted from the lamp may degrade the liquid crystal light valve.
As an alternative to this discharge lamp, a projection type image display device using a laser light source for emitting monochromatic light is proposed here.
However, while a laser light source does not have the problems mentioned above, the laser light source has the disadvantage that it has coherence.
As a result, interference band appears as speckle noise on the projection surface on which laser light is projected and the image deteriorates. Thus, measures against speckle noise are necessary if highly accurate images are to be displayed.
Devices in which dispersion elements are disposed on the projection surface or in an optical system projecting laser light on the projection surface, and devices that vibrate the dispersion elements are well known as devices for eliminating speckle noise.
For instance, an image display device including such a device is proposed in the Japanese Unexamined Patent Application, First Publication No. 2004-144936.
This image display device includes diffusion elements on the surface of which uneven portions of specific shape are formed.
These diffusion elements are made to vibrate at a predetermined frequency by an exciting device in the plane perpendicular to the optical axis, thereby changing the strength distribution of laser light incident onto the diffusion elements.
As a result, the speckle noise can be suppressed.
Moreover, a high output laser is necessary in the light source of a high intensity projector, and an array of light sources is used for high output.
For example, in the Japanese Unexamined Patent Application, First Publication No. 2004-144794, a method of controlling the temperature of the array of light sources is proposed as a measure against the speckle noise due to the array of light sources.
Each semiconductor laser in this array of light sources is maintained at a different temperature in this image display device. Each semiconductor laser emits luminous flux of varying wavelength depending on the difference in temperature.
In this way, by varying the output wavelength, the coherence of the semiconductor lasers can be reduced, and the speckle noise of the entire output source can be reduced.
The image display device disclosed in the Japanese Unexamined Patent Application, First Publication No. 2004-144936 has only one light source. If a plurality of light sources like an array of light sources is used, light is diffused by identical diffusion elements, so the coherence of light beams emitted from multiple light sources can not be reduced.
Also, to vibrate the diffusion elements, an exciting device is used. Therefore, the device on the whole increases in size, and noise due to vibration is generated.
The image display device disclosed in the Japanese Unexamined Patent Application, First Publication No. 2004-144794 is based on the prerequisite that a light source that does not require an external resonating structure, that is, a light source that outputs laser light directly, is used.
Evidently, the speckle noise can be effectively suppressed in case of a light source that does not require an external resonating structure.
Here, the basic configuration elements of a light source including external resonator are the light emitting element and wavelength selective element (resonator mirror).
Even if a plurality of light emitting elements is used, a wavelength selective element that selects a single wavelength is generally used considering the cost benefits and the ease of assembly.
For laser oscillation in the wavelength selective element, the band of the wavelength selected needs to be narrow.
The result is that even if the wavelength of each light beam emitted from an array of light sources is varied, as disclosed in the Japanese Unexamined Patent Application, First Publication No. 2004-144794, a single wavelength is selected by the wavelength selective element, and the coherence of the entire light source including wavelength selective element does not decrease.